It is known to provide a conductive stub in a through-hole formed in the circuit board. The resulting component is commonly referred to as plated through-hole (PTH) via. These are commonly used in circuit boards to allow transfer of signals between circuit board layers. They are also used for connecting selected circuit board planes. Vias allow for beneficial signal routing densities and layer counts. Further, they are relatively easy to manufacture and cost effective—particularly in cases where aspect ratios are below 10:1.
Potential problems arise where vias are used in relatively thick circuit boards—such as backpanels—or in circuit boards carrying signals of relatively high frequency. An unused portion of a via—or more specifically a via stub—often behaves as a transmission line in parallel with the signal transmission path. This is known to cause unwanted electrical effects that impact on signal quality. Further, the signal transmission path is often loaded in an undesirable manner at higher signal frequencies.
A known solution is to reduce the length of the subs. For example: back-drilling to remove unused portions of the stubs. It is also possible to use blind or buried vias as an alternative. These approaches are effective, but costly in manufacturing and create a need for increased efforts in circuit board design. Further, back-drilling is prone to errors which irreversibly damage boards. For example, drill mis-registration, unsatisfactory drill depths, and slanted drilling. Such errors adversely affect manufacturing yield.